Humidity problems are often produced in cold spaces ventilated by exterior air, often only due to the humidity of the surrounding air and the fact that non-heated spaces often are as cold as, or even colder than, their surroundings and they will therefore accumulate humidity to levels wherein mould, fungi and other microorganisms can grow. The problems are aggravated if, in addition thereto, there are internal humidity sources such as for example leakage of humid air into an attic space from the living area. These humidity problems both lead to health problems and to large economical damages.
In order to facilitate the presentation below, some definitions are here given:
Absolute humidity is, conceptually seen, the amount of water in vapour phase that a given volume of air contains, normally expressed in grams per cubic meter, i.e. the density of the water vapour.
Vapour pressure here means the partial pressure of the water vapour in air in agreement with the praxis of the construction field. It should thus be noted that the definition of vapour pressure is different from the established definition within for example organical chemistry (wherein the vapour pressure instead means the pressure at which the evaporation of a substance is in equilibrium with its liquid and solid phases at a certain given temperature).
The saturation vapour content at a certain temperature is the amount of water in vapour phase that air maximally can contain before the water vapour starts to condensate at a surface or starts forming particles in the air.
The relative humidity is the relation between current absolute humidity and the saturation vapour content at the current temperature. This is at a given temperature numerically the same as the relation between the vapour pressure and the saturation vapour pressure, cf. for example Nevander, Elmarsson “Fukthandbok: Praktik och Teori”, Svensk Byggtjänst 1994.
An arrangement has been described (WO 2009/038534 A1) by means of which a crawl space is ventilated with outdoor air only when the absolute humidity is higher indors than outdoors. An arrangement of this type is of course just as applicable at a cool attic space. Such an arrangement can be provided with a heating element which is triggered depending on the risk for mould growth and an attic or a crawl space is protected from damages caused by mould. The technique has been found to function well, particularly the combined solution comprising both absolute humidity controlled ventilation and heating, in those cases where the ventilation is not enough to obtain a climate which is mouldsafe. In connection with the fact that there, as mentioned above, also may be internal humidity sources, such as for example leakage of humid air into an attic space from the living area, it has sometimes been shown to be difficult to, despite the technique described in the cited document, avoid spotwise damages as a consequence of a locally occurring too high absolute humidity, which, in spite of the arrangement, may provide conditions such that for example growth of mould and fungi is promoted. In order to eliminate also this problem an arrangement has been constructed, which in one unit provides all of the so far discussed solution in a package, and which furthermore contains a fan for circulation of the air contained in the space also when there is no ventilation with exterior air. The mixing of air that thereby is obtained, eliminates the above mentioned problems efficiently.
The control of when ventilation with exterior is to take place or not is performed, as earlier described, using information from humidity sensors, wherein the respective relative humidity is measured by means of a sensor at the inside and a sensor at the outside.
It is customary to somewhat carelessly use the expression humidity sensor. Even if this is a functionally descriptive term, it actually does not say anything about what is actually measured. However, normally the temperature and the relative humidity, Hrel, are measured, which values used together give the absolute humidity.
The relations are as follows:Hrel=absolute humidity/saturation vapour content (applicable as a general rule).
Since the saturation vapour content is a temperature dependent quantity it is available as a table value. Reconstruction of the above given relation gives:Absolute humidity=Hrel×saturation vapour content.
Humidity sensors can in other words be described as something that measures the temperature and the relative humidity and something that, in some cases, from these values, extracts a signal corresponding to the absolute humidity.
A property of humidity sensors is that they have a long term stable precision but that they tend to drift in accuracy over time. The effect thereof is that previous systems either have required a regular calibration or their functioning has been poorer than what would have been possible with accurately calibrated sensors. So far it has not been possible to provide a system for measuring humidity in a ventilated space with a satisfactory accuracy. It has also been impossible to provide a system which enables provisioning of reliable measurement data on relative humidity over time and, in general, a system which is flexible.